1. Field of the Invention
The present invention relates to medical devices for implanting into an animal, such as cardiac stimulation devices, and more particularly to such medical devices that receive radio frequency signals via an antenna.
2. Description of the Related Art
A remedy for people with slowed or disrupted natural heart beats is to implant a cardiac pacing device which is a small electronic apparatus that stimulates the heart to beat at regular intervals. That device consists of a pulse generator, implanted in the patient's chest, which produces electrical pulses that stimulate heart contractions. Electrical wires extend from the pulse generator to several electrodes placed nearby specific muscles of the heart, which when electrically stimulated produce contraction of the adjacent heart chambers.
It is quite common that the wires extend through arteries or veins which enter the heart so that the electrodes can be placed in the muscle of the heart chamber requiring stimulation. The wires typically extend for some distance in the arteries or veins and may pass through one or two heart valves. In other patients, patch electrodes are placed on the exterior heart surface with wires extending through tissue to the pacing device. With either type of wire placement, it is important that the electrodes be attached at proper positions on the heart to stimulate the muscles and produce contractions. Thus, it is desirable to properly locate the electrodes for maximum heart stimulation with minimal adverse impact to other physiological functions, such as blood circulation.
More recently wireless pacing devices have been proposed, such as the one described in U.S. Pat. No. 6,445,953. With this type of device, a radio frequency (RF) signal is transmitted from a conventional pacing circuit to stimulator devices placed on the heart at locations where stimulation is to occur. For example, the stimulator device can be implanted in a blood vessel of the heart. The radio frequency signal activates the device which applies an electrical pulse to the heart tissue. Electrical power for stimulating the heart is derived from the energy of the radio frequency signal.
One of the difficulties in such a wireless system is ensuring that the radio frequency signal and a maximum amount of the RF energy is received by the stimulator device. In the case of that prior patented device, the antenna was a coil wrapped around a cylindrical structure that was embedded against the wall of a vein or artery. That type of antenna received the greatest amount of energy from an electromagnetic field oriented in a direction through the turns of the coil. However, since the antenna can be implanted in different orientations in the patient's body depending on the location of the vein or artery and the orientation of the transmitter antenna similarly varies, it is difficult to ensure that the electromagnetic field from the RF signal will be properly aligned with the antenna of the implanted device. Because choosing the location at which the medical device in implanted is based primarily on cardiac stimulation criteria, it is not always possible to optimally orient its antenna for maximum energy reception.
A proposed transmitter, for sending signals to the implanted medical device, employs an omnidirectional antenna which emits electromagnetic waves that propagate along three orthogonal axes. Thus one of those electromagnetic waves or a vector sum of two or all of them will be aligned with the antenna coil of the implanted medical device. Although this solves the problem of misalignment of the transmitter and receiver antennas, the RF energy from all the electromagnetic waves is not received by the implanted device. Since that device is powered by the RF energy, it is desirable to receive as much of the transmitted energy as possible. That desire is especially acute when the medical device is an implanted defibrillator as such apparatus requires a relatively large amount of power to defibrillate a heart.